A study shows that although box jellyfish only have a simple nervous system, they can learn from experience and adapt their behavior sensibly, experiments show. The study thus calls into question the previous assumption that a central brain is necessary for more complex forms of learning. The results also provide clues to the evolutionary roots of learning and memory functions in the animal kingdom, say the researchers.
Experience makes you wise: This is an important recipe for success for our species. Because we can grasp connections in a particularly sophisticated way, remember them and then use what we have learned sensibly. As is well known, many animals also have this ability for so-called associative learning in various forms. Until now, however, it was assumed that this required a more highly developed brain, such as that possessed by vertebrates, insects and some molluscs. However, “simpler” creatures such as jellyfish have so far been denied this ability. It was assumed that their rather simple nervous system could only convert sensory impressions into reactions in a primitive way.
Supposedly simple creatures in their sights
A German-Danish research team has now questioned this previous assumption experimentally. Caribbean box jellyfish (Tripedalia cystophora) served as experimental animals. Only the size of a fingernail, these cnidarians pulsate through the murky waters of mangrove swamps on the hunt for water fleas. For this purpose, its screen is equipped with 24 eyes, which are connected to four structures, each made up of around a thousand nerve cells. Using their sense of sight, the sensitive jellyfish can detect obstacles such as underwater roots and avoid them by adjusting their umbrella contractions.
As the scientists explain, the box jellyfish perceive the spatial distance to the obstacle through contrast: at a certain level of darkness caused by the approaching root, they move away to avoid a collision. The researchers have now investigated the question of whether a learning process also plays a role in this behavior. This seemed conceivable because visibility in the water can change significantly due to variations in the suspended matter content in the box jellyfish’s habitat.
Box jellyfish also become wise through experience
To simulate their habitat, the researchers conducted their experiments in a test tank whose interior walls had a pattern of gray and white stripes. The light areas corresponded to the open water environment and the dark stripes represented the mangrove roots that had to be avoided. Box jellyfish were then placed in the test tank at different contrast ratios and the researchers then analyzed the swimming behavior of the animals.
This showed that at the beginning of the experiments, the jellyfish often bumped into the simulated roots on the tank wall. But after just a few minutes, they had already increased their average distance from the obstacles by around 50 percent and, by adjusting their swimming direction, they only crashed into the dark areas half as often. It also showed that when the researchers changed the contrasts, the animals adapted to these apparently changed visibility conditions.
“The results showed that box jellyfish can learn through the combination of visual and mechanical stimulus experiences,” says senior author Anders Garm from the University of Copenhagen. “We discovered that three to five failed evasive maneuvers were enough to change the behavior of the jellyfish so that they no longer hit the simulated roots. This finding is particularly interesting because it roughly corresponds to the input that a fruit fly or mouse needs to learn,” says Garm.
On the trail of the neuronal process
In order to gain initial insights into the underlying processes, the scientists then carried out studies on the functional units of the system: These are the four rhopalia, each of which includes six eyes and a complex of around 1000 nerves that are connected to the movement system are connected. The researchers now presented dark bars to the eyes of isolated Rhopalia to simulate the jellyfish approaching an obstacle. As it turned out, the structure initially did not react to light gray stimuli from nerve impulses – it apparently interpreted these as obstacles that were still far away. However, after the researchers stimulated the Rhopalium with simulated collisions, it also began responding to the light gray bars with signals to avoid obstacles.
As the team explains, the results now have considerable significance for research: “For neuroscience, the results provide a new perspective on what can be achieved with a simple nervous system,” says Garm. First author Jan Bielecki from the Christian Albrechts University in Kiel also emphasizes the evolutionary significance. Because box jellyfish are probably still similar to the creatures that developed nervous systems for the first time in evolutionary history. “If these animals are able to learn, it could be a fundamental ability of nerve cells or neural networks. “This suggests that this ability existed earlier than previously thought,” said Bielecki.
The team now wants to stay on the ball: “We are now trying to find out more precisely which cells are involved in learning and memory formation,” says Garm. It may be that these are fundamental neural concepts, say the researchers. “In this case, the box jellyfish could develop into a model system to study cellular processes during advanced learning in animals,” the scientist concludes.
Source: University of Copenhagen – Faculty of Science, Christian-Albrechts-Universität zu Kiel, specialist article: Current Biology, doi: 10.1016/j.cub.2023.08.056